This invention relates to a liquid type display cell wherein a flowable substance such as a liquid crystal or an electrochromic substance is sealed between a pair of substrates and a method of manufacturing such a display cell.
As shown by a sectional view shown in FIG. 1, a prior art electrochromic display cell comprises a glass substrate 1 coated with a transparent electroconductive film 2 on which is mounted a thin film 3 of an electrochromic substance, WO.sub.3 for example. A substrate 6 made of stainless steel, for example, is secured to the glass substrate 1 by a spacer 4 made of tetrafluoroethylene, for example, and an epoxy bonding agent 5. On the inner surface of substrate 6 a counter electrode 7 made of carbon or WO.sub.3 is bonded. Liquid ion conductor 8 filling the space between substrates 1 and 7 comprises a mixture of glycerine, sulfuric acid, WO.sub.3 and TiO.sub.2. After pouring the mixture into the space through a pouring port 9, the port 9 is sealed by an epoxide sealing agent 10. When a voltage is impressed across the electrochromic film 3 and the counter electrode 7, the electrochromic film 3 emits coloured rays. However, at the same time voltage is also impressed across the transparent electrode 2 made of SnO.sub.2 and the counter electrode 7, so that current also flows therebetween thus consuming power. For this reason, it is not advantageous to use an electrochromic cell of this type for an instrument requiring extremely small power consumption such as a wrist watch. Such current also ages the transparent electrode 2 with the result that the electrode 2 becomes opaque and evolves gas.
As described above the prior art liquid crystal or solution type electrochromic display cell is constructed to allow pour liquid to be poured into a space between two substrates and then hermetically seal the cell. Although inorganic sealing agent such as low melting glass is preferred because of its reliability, as the temperature for applying the sealing agent exceeds 400.degree. C. so that the electrodes applied to the substrates and the electrochromic substance are required to withstand against such high temperature.
Where a substance having a low heat resistance property is used for the liquid crystal or solution type electrochromic display cell, it is necessary to use a sealing agent having lower working temperature, for example, an organic bonding agent. However, where the substrates and the liquid pouring port are sealed by such organic sealing agent, it is difficult to perfectly seal the pouring port in an air tight fashion.
FIGS. 2 and 3 are a sectional view and a plan view respectively of a prior art electrochromic display cell utilizing seven segment electrodes wherein a transparent glass substrate 1A is provided with seven electrodes 2A. Lead wires 3A, and a layer of electrochromic substance 4A having an area a little larger than the area of seven electrodes is applied to cover these electrodes. A transparent insulating layer 5A is formed on the lead wires 3A by vapor depositing SiO.sub.2 through a mask. A counter electrode 7A consisting essentially of carbon is bonded to the inner surface of a lower substrate 6A. A spacing ring 8A is made of an epoxy resin, and an electrolyte 9A is filled in the space between the upper and lower substrates through a pouring port, not shown. In the case of FIG. 3, five electrode segments are shaded to display digit 5. During operation five, segments 10A are colored but the peripheries of not used segments 11A are also coloured.
To manufacture such an electrochromic display cell provided with seven electrode segments which are selectively energized to display different digits, a transparent electrode consisting essentially of In.sub.2 O.sub.3 or SnO.sub.2 is applied onto the entire surface of a transparent glass substrate by the vapor deposition technique. Seven electrode segments and lead wires are then formed by photoetching technique. Then, electrochromic substance such as WO.sub.3, MoO.sub.3, mixtures thereof, doped or not doped with other elements is vapor deposited on the electrode segments through a mask provided with perforations corresponding to the electrode segments. On the lead wires a transparent insulating layer is vapor deposited. A counter electrode is formed on the lower substrate. The transparent glass substrate and the lower substrate are placed to oppose each other such that the electrochromic substance and the counter electrode face each other. The peripheries of the upper and lower substrates are sealed and spaced. Thereafter, an electrolyte is poured into the cell and the pouring port is sealed to complete the cell. However, it is extremely difficult or practically impossible to perfectly align the electrode segments formed by photoetching technique and the electrochromic substance vapor deposited through a mask.
The perforations of the mask is slightly enlarged so that the sizes of the electrochromic substance layers will be slightly larger than the sizes of the electrode segments. Although the layers of the electrochromic substance overlying the electrode segments become perfectly chromatic and nonchromatic, the portions of the layers of the electrochromic substance overhanging the electrode segments become perfectly chromatic but their discoloration is not perfect so that after about 1000 operations these overhanging portions form colored edges.
As described above, in an electrochromic display cell having a construction discribed above and manufactured by the above described method, a serious problem of forming colored edges exists. Especially where small display cells are required as in wrist watches, other problems such as vapor deposition to undesired portions and the limits on the accuracy of patterning are also encountered.